Mobile terminals, such as cellular phones, are useful communication tools. However, cellular phones, such as smart-phones, may have a relatively large number of functions, and a battery powering the cellular phones may run out unexpectedly when various functions are utilized for a relatively long period of time, for example. When the battery runs out, the functions of the cellular phones may not be used unless the battery is replaced or the cellular phone is directly connected to a power supply.
Recently, there is active research and development in wireless power transfer. The wireless power transfer is also sometimes referred to as contactless power transfer. For example, by using the wireless power transfer, it may be possible to charge a battery of a mobile terminal, such as cellular phone, by merely placing the mobile terminal against or on a charging unit without physically and electrically connecting the mobile terminal to a power supply. However, since the charging of the battery does not occur unless the mobile terminal is placed close to the charging unit, this wireless power transfer may be unsuited for use during motion of the mobile terminal. The motion of the mobile terminal occurs when a user, a vehicle or the like carrying the mobile terminal moves.
Compared to the wireless power transfer described above having a power transfer distance on the order of several mm, for example, a first wireless power transfer system has been proposed in which the power transfer distance may be extended to 2 m, for example. This first wireless power transfer system is based on the principles of resonance using coils in order to enable an efficient power transfer. However, since the coil size needs to be increased in order to extend the power transfer distance, this first wireless power transfer system may be unsuited for implementation in portable devices, such as mobile terminals. In addition, in order to realize an efficient power transfer, a power transmitter in a transmitting end and a power receiver in the receiving end device may need to be oriented to oppose each other, which may limit the arrangement of the power transmitter, for example. Furthermore, the need to orient the power transmitter and the power receiver to oppose each other may make it difficult to transfer the power according to this first wireless power transfer system during motion of the mobile terminal.
On the other hand, a second wireless power transfer system has been proposed in which the power is transmitted and received by means of microwaves. As opposed to the first wireless power transfer system described above, this second wireless power transfer system may not limit the arrangement of power transmitter, for example, and the power receiver may receive power even during motion of the mobile terminal. However, according to this second wireless power transfer system, the power transfer efficiency may be considerably reduced compared to that of the first wireless power transfer system unless the beam width of either a transmitting antenna or a receiving antenna is narrowed. But narrowing the beam width of the transmitting or receiving antenna may make it more difficult for the mobile terminal to receive the microwaves during motion.
Moreover, according to the first and second wireless power transfer systems, the cost of setting up the power transmitters in an environment in which the mobile terminal is used may become considerably high when a relatively large number of power transmitters are to be set up in the environment.
Therefore, there may be demands for a wireless power transfer technology that may realize an efficient power transfer even during motion of the mobile terminal.